QPainterPathStroker: make QPen constructor explicit

A QPainterPathStroker is not an equivalent representation of a QPen, so
the constructor that takes a QPen should be explicit.

Arguably, the named constructor idiom would be even better here:

    static QPainterPathStroker QPainterPathStroker::fromPen(const QPen &pen);

But QPainterPathStroker is non-copyable.

Change-Id: I3148dc0ee336026781d8bc1baf21c113c7b41ce8
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

src/gui/painting/qpainterpath.h

@@ -244,7 +244,7 @@ class Q_GUI_EXPORT QPainterPathStroker
     Q_DECLARE_PRIVATE(QPainterPathStroker)
 public:
     QPainterPathStroker();
-    QPainterPathStroker(const QPen &pen);
+    explicit QPainterPathStroker(const QPen &pen);
     ~QPainterPathStroker();
 
     void setWidth(qreal width);